For the production of smoked sausage products, hitherto principally casings based on regenerated cellulose or collagen have been used. The production of these casings is technically complex, however. For instance, cellulose casings are as a rule made by the viskose method. In this method, celluloses first converted to cellulose xanthogenate using sodium hydroxide solution and carbon disulfide (CS2). What is termed viscose solution which results must first mature for a plurality of days before it is fed to the skin spinning machines. These machines essentially comprise a spinning die, precipitation baths, wash and preparation baths and also drying stations. In the precipitation baths, the cellulose xanthogenate is regenerated to form cellulose. Collagen skins, also termed hide fiber skins, comprise cured connective tissue protein. In their production, first connected tissue from animal hides is mechanically comminuted and chemically digested. The resultant homogenized mass is then further processed in a dry- or wet-spinning process. In the wet-spinning process, the collagen mass, after extrusion through a ring die, is solidified in a coagulating precipitation bath (G. Effenberger, Wursthüllen-Kunstdarm [Sausage casings, artificial skins], Holzmann-Buchverlag, Bad Wörishofen, 2nd Edition [1991] pp. 21-27).
Cellulose casings and collagen casings are very highly permeable not only to smoke, but also to water vapor. The permeability is generally more than 500 g/m2·d. Owing to the high water vapor transmission rate of the casing, however, the sausage dries out in an unwanted manner when it is stored for some time.
A further disadvantage of collagen casings and cellulose casings is the high production costs owing to said wet-chemical processes.
For unsmoked sausage products, currently inexpensive casings made of thermoplastics are widely used. Customary plastics are polyamides, polyesters and vinyl chloride copolymers. The casings can be single-layered or multilayered. In the multilayered casings, frequently polyolefin layers are also present. The decisive advantage of these casings is the technically relatively simple and inexpensive production. Casings made of thermoplastic have a water vapor transmission rate (WVTR) of about 3 to 20 g/m2·d. They are thus markedly less permeable than casings made of regenerated cellulose or collagen. Sausage products in such a casing therefore loose markedly less weight during storage. Casings produced from thermoplastics, for instance from polyamide, have to date been generally considered not to be permeable to smoke and therefore not smokeable.
However, some smokeable plastic casings are known. For instance, in EP-A 139 888 discloses a method for smoking foods in a casing made of aliphatic polyamide. The polyamide absorbs at least 3% by weight, preferably at least 5% by weight, of water. Smoking is therefore carried out in the presence of water or water vapor, which requires a climatically controlled smoking cabinet.
A smokeable film for packaging foods is also described in EP-A 217 069. It comprises at least one layer which comprises a mixture of polyamide, an ethylene/vinyl alcohol copolymer (EVOH) and polyolefin, the constituents of the layer being in a defined weight ratio. The layer has a water vapor transmission rate of less than 40 g/m2·d at a temperature of 40° C. and a relative air humidity of 90%. However, this is only a slight increase compared with the abovementioned range of values for thermoplastic casings. Correspondingly, under customary conditions, no satisfactory smoke permeation is to be expected either.
The object is still to provide a plastic casing which has a very good permeability to smoke without special conditions (defined air humidity, defined temperature, etc.) leading to the net during smoking. In particular, intense transfer of color and flavor to the sausage meat emulsion is to be possible not only using hot smoke (about 70 to 80° C.; particularly for scalded-emulsion sausage), but also using cold smoke (about 20 to 35° C., particular for cooked-meat sausage and raw sausage). Those skilled in the art are familiar with the fact that the diffusion of gases through plastics is highly temperature-dependent. Thus the use of cold smoke a priori requires a higher permeability of the plastic than does that of hot smoke.
The casing, furthermore, it to be inexpensive and simple to produce by the thermoplastic route. It is to be smoke-permeable and resistant even to hot water and water vapor, so that it can also be used in the production of scalded-emulsion sausage. The water vapor transmission rate of the casing (determined as specified in DIN 53 122) is to be at least 40 g/m2·d, but no more than 200 g/m2·d with unilateral impingement of the casing with air which has a temperature of 23° C. and a relative humidity of 85%, so that the foods situated in the casing dry out as little as possible after smoking.
The object is achieved by a casing of a mixture which comprises aliphatic polyamide and/or aliphatic copolyamide, at least one heat-plasticizable, water-soluble organic polymer and also if appropriate additives of other organic or inorganic substances. “Water soluble” in the context of the present invention is taken to mean polymers the solubility of which in water at 80° C. is at least 20 g/l.
Water-soluble synthetic polymers, in particular poly(vinyl alcohol) (PVAL), have already been used for a relatively long time for producing certain semi-permeable membranes. The membrane production is performed in principle from solutions. The film resulting from a precipitation process must then be crosslinked to abolish the water solubility.
Little is known to date of thermoplastic deformation of water-soluble polymers. Owing to its high polarity and the associated intermolecular interactions, its melting point is usually markedly above the decomposition temperature. Articles made of water-soluble polymers, moreover, would have to subjected to a subsequent cross-linking treatment, in order to make them moisture-resistant in the later application.
Surprisingly, it has been found that blends of aliphatic (co)polyamide and water-soluble polymer                are water resistant without post-crosslinking, i.e. that virtually no water-soluble polymer is dissolved out by cold or hot water,        are extrudable without decomposition and can be shaped to form tubular films,        they may be processed by means of blown film processing methods to form food casings having advantageous mechanical properties and        have water-vapor and smoke permeabilities in the above required range.        
Blends of polyamides and water-soluble polymers are in principle already known. The relevant publications, however, do not refer to the field of application of the present invention and in addition describe other properties.
WO 94/16020 describes biodegradable blends of two polymers, of which each is to be already biodegradable. As first polymer, mention may be made of, inter alia, polyamide (PA), as second component, inter alia, PVAL, polylactide and other aliphatic polyesters. Optionally, in addition, a polysaccharide is to be admixed. In the examples, predominantly blends of EVOH and PVAL are listed. If films made from blends are treated with hot water, the PVAL is extracted from the matrix. The said blends are thus unsuitable for producing therefrom films for encasing moist foods.
WO 94/03544 discloses blends made of a water-soluble polymer and a water-insoluble matrix polymer for producing medical articles. The water-soluble polymer is a poly(ethylene glycol), a poly(ethyloxazoline), a poly vinyl(alcohol), a polyacrylamide, a polyvinylpyrrolidone or a polyacrylic acid. The matrix polymer, in turn, is an ethylene/vinyl acetate copolymer (EVA), a polyolefin, a poly(vinyl chloride) (PVC), a polystyrene, a polystyrene/butadiene copolymer, a polycarbonate, polyacrylate, a polyamide or a copolymer thereof, a polyurethane, a polyester or a copolymer thereof. The surface of the articles is hydrophilic and protein-compatible, which decreases, in particular, the absorption of blood protein.
JP-A-62-132952 discloses thermoplastic blends of 20 to 80% by weight of a polyamide and 80 to 20% by weight of PVAL and also articles produced therefrom, for example films and tubes, which are distinguished by a decreased electrical surface resistance and as a result improved antistatic properties.
U.S. Pat. No. 4,611,019 relates to a blend of thermoplastic poly(vinyl alcohol) homopolymer having a degree of saponification of more than 95%, a plasticizer and a small fraction (0.5 to 4.5% by weight, based on PVAL) of polyamide or polyester. Compared with pure PVAL, the blend has the advantage of an improved oxygen barrier, in particular at high humidities (i.e. at more than 75% relative humidity). This observation opposes the inventive object in which permeation of (smoke) gases is actually desired.